1. Field of the Invention
The present invention relates to a method and apparatus for measuring properties of fluids, in particular the gel strength and/or density of drilling fluids.
2. Description of the Prior Art
In the drilling of wells by rotary drilling techniques, a drilling fluid, referred to as mud, is circulated from the surface through a drill string and back to the surface. The mud serves several important functions in the drilling operation, two of the most important of which are maintenance of hydrostatic pressure on subsurface formations and the suspension and removal of drill solids from the well. To achieve these functions, the mud must be maintained within a carefully controlled density range and gel strength range. The density and gel strength properties of the mud, therefore, are constantly monitored during the drilling operation.
Over the years several instruments for measuring the mud density and gel strength have evolved. Instruments for measuring density range in complexity from the simple mud balance to the rather sophisticated differential pressure mud weight instrument. One prior differential pressure mud weight instrument is sold by Samega under the trade name of Densimeter DMC and is disclosed on Page 5719 of the Composite Catalog of Oil Field Equipment and Services, 33rd Revision (1978-79), published by World Oil. Other differential pressure measurement instruments are also disclosed in U.S. Pat. Nos. 2,451,604, 3,175,403, 4,059,744 and Dutch Pat. No. 7,407,114. Differential pressure mud weight instruments generally measure the difference in pressure between two vertically spaced points in mud flowing through a conduit. The pressure differential provides an indication of mud density. Although this type of instrument represents an improvement over the mud balance, it still has certain disadvantages. The instrument does not distinguish between hydrostatic pressure and friction pressure and therefore, measurements under certain conditions will indicate densities substantially different from actual densities. Further, differential pressure mud weight instruments are sensitive to the presence of gas in the mud. It is known that gas entrained in the mud has an effect on mud density. At the surface, the gas-cut or aerated mud density would be less than the mud density in the well under pressure. From an operational standpoint, for mud which is to be pumped into a well, the density of the mud under pressure is more meaningful because it more accurately represents the actual mud density in the well and, hence, the hydrostatic pressure exerted by the mud. The effects of air on mud density may be even more pronounced when certain lost circulation materials such as straw or other fibrous matter are present in the mud because these materials tend to entrain air.
It is also often desired to measure the density of mud as it returns from the well in order to detect the presence of gas or other formation fluids. In this instance, it is generally desirable to measure the density at atmospheric pressure.
Other instruments for measuring mud weight or density are disclosed in U.S. Pat. No. 2,609,681 and Canadian Well Logging Society Paper No. 7065 presented in Calgary, May 6-8, 1970.
Gel strength, defined as the property of a mud to develop and retain rigid form, is an important measure of the mud's ability to suspend drilled solids in a quiescent condition. The gel strength should be sufficiently high to suspend the solids, but not so high as to retard drilling operations. A common instrument for measuring gel strength is the Fann V-G meter. This instrument, however, does not provide a continual record or comparison of the mud gel strength at frequent time intervals. U.S. Pat. No. 3,069,900 discloses apparatus and method for measuring mud properties (viscosities and gel strength) of non-Newtonian liquids. Another type of instrument for measuring gel strength and viscosities is disclosed in British Pat. No. 1,280,227. The gel strength instrument generally comprises a horizontally disposed pipe loop having two pressure taps located along its length. The mud is introduced into the loop and allowed to set for a predetermined time. Flow is then initiated and the peak pressure difference between the two taps is directly proportional to the gel strength.